class_docstr: An implementation of exact Entmax with alpha=1.5 (B. Peters, V. Niculae, A. Martins). See
  :cite:`https://arxiv.org/abs/1905.05702 for detailed description.
  Source: https://github.com/deep-spin/entmax
code: def backward(ctx, grad_output):
      (Y,) = ctx.saved_tensors
      gppr = Y.sqrt()  # = 1 / g'' (Y)
      dX = grad_output * gppr
      q = dX.sum(ctx.dim) / gppr.sum(ctx.dim)
      q = q.unsqueeze(ctx.dim)
      dX -= q * gppr
      return dX, None
  
imports: from torch import nn
  from torch.autograd import Function
  import torch.nn.functional
  import torch
label_desc: Use image node_id 2 for calling the Entmax15Function obj's underlying member method code with example usage: obj.backward(ctx, grad_output) and returns: dX,  None